Totally depends on the make up of the decals
A dry transfer, as a Lloyds use regular, needs a clear coat over the top, if it’s a self adhesive vinyl based decal, a little thicker, it doesn’t have to be clear coated but durability can be improved if you do.
However use a 2 pack or air dry clear coat as the high temperatures used to cure some clear coats will effect the vinyl decals
I do 20 or so sets of vinyl decals every week and have been doing so for years, some are clear coated, some are just straight over the top of a powder, I would imagine it’s the oven temperature for the clear coat that’s the issue 😉